Ahh the power of tooling. Could this be the beginnings of "Microsoft Visual Game Studio 2010: RAD Edition"? :-P
But seriously ... I think there's a lot that can be done on the tooling side. For example, it wouldn't be too far-fetched to imagine creating a custom UITypeEditor for your content pipeline processors that spin up an XNA Graphics Device and let you edit in real time resources like heightmaps, shaders, or maybe even let you preview model material changes in real-time.
Cool Stuff :-)